Currently, spin valves are typically used for the magnetoresistive (MR) element in MR read heads. A spin valve includes two magnetic layers, a free layer and a pinned layer. A spin valve also includes a spacer layer and a conventional antiferromagnetic (AFM) layer. The spin valve may also include a capping layer. The free layer and pinned layer are separated by the spacer layer. The magnetic moment of the pinned layer is typically fixed by exchange coupling to the conventional AFM layer. The free layer is typically biased to ensure that the free layer has a single domain structure. The free layer is typically biased using either permanent magnets or antiferromagnets. In order to read data stored on a recording media, the MR head is placed in proximity to the recording media. The magnetic moment of the free layer may change based on the magnetic moment of the bit currently being read. As a result, the resistance of the spin valve changes and the magnetic moment of the bit can be determined.
The current trend in recording technology is toward higher areal densities. As densities increase, the size of each bit is reduced. Therefore, the magnetic moment of each bit diminishes. As the magnetic moment of each bit decreases, the ability of each bit to affect magnetic moment the free layer is reduced. This is because the magnetic moment of the free layer becomes very large compared to the magnetic moment of a bit. Because the bit has less effect on the free layer, the spin valve becomes unable to read efficiently at higher density recording media.
In order to increase the ability of the free layer to read at higher densities, the magnetic moment of the free layer is reduced. If the magnetic moment of the free layer is reduced by decreasing the thickness of the free layer, an increased portion of the resistance of the spin valve is due to scattering at the surfaces of the free layer. This is particularly true when the free layer has a thickness that is less than the mean free path of electrons in the free layer, which is approximately fifty Angstroms. This scattering at the surfaces of the free layer is spin independent and, therefore, reduces the portion of the resistance of the spin valve that is due to magnetoresistance. Decreasing the thickness of the free layer thus reduces the magnetoresistance of the spin valve, which is undesirable.
To avoid this reduction in magnetoresistance, the magnetic moment of the free layer can be reduced by providing a ferrimagnetic synthetic free layer. The synthetic free layer includes two ferromagnetic layers separated by a spacer layer. The distance between the ferromagnetic layers is selected so that the ferromagnetic layers are strongly antiferromagnetically coupled. Furthermore, one ferromagnetic layer has a greater magnetic moment than the other ferromagnetic layer. Thus, although the ferromagnetic layers are antiferromagnetically coupled, the combination has a small magnetic moment. This allows the spin valve having the synthetic free layer to be used in reading higher densities. Furthermore, the thickness of the synthetic free layer can be set so to be longer than the mean free path of elections in the layer. Thus, the symmetric free layer maintains a physical thickness greater than fifty Angstroms, while decreasing the effective magnetic thickness. Scattering at the surfaces of the synthetic free layer does not unduly reduce the magnetoresistance of the spin valve. Thus, the spin valve having the synthetic free layer can be used in reading higher areal density recording media.
Although the spin valve having a synthetic free layer can be used in high density recording applications, the magnetic moment of the synthetic free layer may be unstable. It is desirable for each of the ferromagnetic layers of the synthetic free layer to have a single domain structure. If the ferromagnetic layers in the synthetic free layer have multiple domains, then the walls of the domains will move when the spin valve is used to read data. Domain wall motion is a source of non-repeatable noise. Noise due to domain wall motion can be substantially eliminated if the magnetic layers in the synthetic free layer have a single domain structure.
Typically, permanent magnets are used to magnetically bias a synthetic free layer of a spin valve. However, if permanent magnets are used to bias the synthetic free layer, then the permanent magnets are magnetostatically coupled to both of the ferromagnetic layers in the synthetic free layer. The ferromagnetic layers have opposite directions of magnetization. The ferromagnetic layer having a direction of magnetization that is the same as the permanent magnet may have a single domain structure. However, the permanent magnet may cause domains to be formed in the other ferromagnetic layer. Consequently, permanent magnets do not ensure that both ferromagnetic layers of the synthetic free layer have a single domain structure. Therefore, noise due to domain wall motion is not reduced.
Accordingly, what is needed is a system and method for biasing the synthetic free layer. The present invention addresses such a need.